Firstly, a system will be described in which analysis of the internal properties of the article to be measured is conducted using for example a spectrophotometer of the front-separation type: in this system, first of all, light from the light source is divided into monochromatic light by for example a diffraction grating and is then simultaneously directed onto the article to be measured and a reference article, using for example a half-mirror. Spectral data is then obtained by receiving the respective reflected or transmitted beams by the photodetection elements, performing respective logarithmic amplification on the signals of the photodetection elements where the light is thus received, and performing differential amplification on the outputs of these, to obtain the absorbancy (OD), the analogue quantity obtained by logarithmic conversion being subjected to A/D conversion to obtain spectral data.
Secondly, a spectrophotometer of the online type which is in common practical use will be described: in this spectrophotometer, since articles to be measured such as fruit or vegetables differ considerably in their optical transmissivity in accordance with for example their type or size, ripeness, skin or density, either the exposure time or the frequency of the drive clock of for example a charge accumulation type line sensor is adjusted in accordance with the size of the articles to be measured as they are fed, or the gain of the amplification unit circuit is set beforehand such that the amount of transmitted light is not saturated even when transmitted by the largest article to be measured. For example, when for example the weigh-in line is altered in accordance with for example change in the type of article to be measured, the gain may be set at the same time as alteration of the type of article to be measured and the spectral data may be obtained by A/D conversion of the analogue quantity output with prescribed (fixed) gain, set for all cases, whilst articles to be measured which are of the same size are measured.
Considering the fact that a logarithmic amplification unit is employed in the above spectrophotometer, the following problems may arise:                (i) A zero-point correction circuit for the pixel units when the photodetection element group is driven must be inserted upstream of the logarithmic amplification unit, and this makes it difficult to cancel all of the noise of the sensor amplification circuit system, including the logarithmic amplification unit.        (ii) if we assume:                    (I) (OD)=LOG(target)−log(reference)            (II) subtraction of offset when perfectly screened                        since (I) and (II) are factors that cannot be applied simultaneously, operation cannot be performed using analogue signals. Accordingly, the target offset subtraction calculation must be performed by quantization (A/D conversion) and digital anti-logarithmic operation: this is a complicated process and of course results in reduced accuracy.        (iii) In quantization of an analogue signal using a logarithmic amplifier, the low-level region of the signal is expanded by logarithmic amplification, but compression is performed in the high level region; consequently, the amount of information in the high-level region is reduced, tending to cause a deterioration in accuracy.        (1) Regarding the method of adjusting the exposure time in accordance with the size of the articles to be measured, the amount of transmitted light changes considerably not merely with size but, in particular in the case of fruit or vegetables, even for the same type of article, with the thickness and density of the skin and ripeness (internal properties) and the like. Also, due to for example the photodetection construction of the feed line, there are restrictions such as that the exposure time has to be kept fixed: thus is not in fact possible to obtain spectral data of optimum level.        (2) There are problems when subtracting the offset data. Specifically, in order to subtract the offset from the target or reference, the anti-logarithms of the respective data must be found before the subtraction is performed. Not only is this process of finding the anti-logarithm of the target data, reference data or offset data troublesome, but, in particular in the online case, electrical offset is produced for example by the dark current of the photodetection sensor or optical leakage resulting from the construction of the feed line, which may result in a multiplication factor of several times, and this electrical offset must be accurately cancelled.        (3) There are problems concerning cancellation of noise contained in the sensor amplification circuit system. Specifically, when the time and accuracy and other factors required for anti-logarithmic operation are taken into account in the logarithmically amplified signal, it is difficult to cancel for example noise i.e. 1/f noise of the amplification system or noise produced by commercial power source frequency components. For example, it is extremely difficult to perform successive zero sampling in the analogue level with high speed and high accuracy, taking into account all the offsets (including noise) of the amplification system prior to sequentially reading the charge accumulated on the pixels of the line sensor.        
When measuring the internal properties including for example sugar content and acidity of fruit or vegetables in a spectrophotometer, the internal properties cannot be directly calculated by digital operation of the various data that are quantized (digitized) by the A/D converter. In this case, first of all, the dark current wavelength data when the light is fully screened is subtracted from the data of the article to be measured and the reference data, and normalization and the like steps are performed, to obtain the ratio of the logarithms and computation such as second-order differentiation is performed on this ratio in order to cancel the offset and gradient, for example, and the difference in optical absorbance characteristic (OD (λ)) with neighboring wavelengths is thereby obtained.
From this difference, the internal properties are calculated by for example statistical processing using multiple regression and P.L.S. etc. Consequently the following problems considerably affect accuracy.
For example in the case of Wenzhou oranges, due to differences in size of the order of 40 mm to 100 mm and differences such as individual density and skin, the amount of light transmitted may typically vary by a factor of about 60: such oranges move along the conveyor in random sequence. In the case of a spectrophotometer of conventional construction, the gain of the amplifier must be set beforehand such that at the maximum point of amount of light transmitted of the spectral data of oranges whose amount of transmitted light is greatest, the output of the amplifier is not saturated and such that the level of the A/D converter is close to the maximum level. Of course, the spectral data of the oranges whose amount of transmitted light is least will have a value of the amount of transmitted light of 1/60 of the oranges whose amount of transmitted light is greatest: thus sufficient measurement accuracy cannot be obtained. As mentioned above, when the internal properties of for example fruit or vegetables are calculated from a difference by statistical processing and the like, such deterioration of measurement accuracy is typically overlooked. In particular regarding the deterioration of measurement accuracy when quantization is performed, a specific description may be given as follows.
FIG. 4 shows the wavelength characteristic of oranges (articles to be measured) whose amounts of light transmitted differ by a factor of 1 to 1/60, due to differences in for example the size or thickness of the skin, when the gain is fixed. The waveform 3A indicated by the continuous line, at the top, indicates the case where the amount of light transmitted corresponds to a multiple of 1; the waveform 3B indicated by the single-dotted chain line, second from the top, indicates the case where the amount of light transmitted corresponds to a multiple of 1/10; the waveform 3C indicated by the broken line, third from the top, indicates the case where the amount of light transmitted corresponds to a multiple of 1/60. For example in the case where the articles to be measured are of the same type and have a transmitted amount of light characteristic of the same form, when two articles to be measured whose amounts of light transmitted differ for example by a factor of 10 are measured by for example the difference of size, if an A/D converter of 12 BIT (0 to 4095) resolution is employed, provisionally assuming that waves having amplitude of values 0 to ±10 reflected by sugar in the vicinity of 920 nm on the spectral data of oranges are superimposed, the A/D converted value in oranges having the largest amount of transmitted light, as indicated by the waveform 3A, is 1000, which, superimposed on the amplitude given above, results in a value of 1010 to 990.
Next, in the case of oranges having a small amount of transmitted light, as shown by the waveform 3B, the A/D converted value likewise in the vicinity of 920 nm is 1/10 thereof i.e. an A/D converter value of 101 to 99 is indicated: a wave having an amplitude of less than 10 superimposed thereon thus represents less than the minimum resolution of the A/D converter and hence is extinguished.
Of course, in the case of articles to be measured whose amounts of light transmitted differ by a factor of 1 to 60, when the optical absorbance characteristic is subjected to analysis processing such as multiple regression or P.L.S. using for example n-th order differentiation, the measurement accuracy deteriorates considerably.
Even if for example a high-resolution A/D converter is employed, there is an appreciable deterioration in the amount of information. Such deterioration of the amount of information as described above must be paid particular regard to when quantization is performed.
In the above, the case was described by way of example in which a fixed gain was set beforehand. However, a spectrophotometer has been proposed (see for example Patent Reference 1) in which there are respectively provided a first detector that detects a sample signal in the optical path on the sample side and an amplifier that amplifies the output signal from this first detector, and a second detector that detects a comparison signal in an optical path on the comparison side and an amplifier that amplifies the output signal from this second detector, comprising means for independently setting the degree of amplification (gain) of these two amplifiers.
Patent Reference 1: Laid-open Japanese Patent Application No. H. 8-101121